For Hera and Zeus, there weren't very many other gods at the time. And they were all related anyway.
Similar with Norse myth as most of them were family in the first place. Incest is common in myth because the Gods exist first and they are very much family in one way or another.
These are also very different cultures to our own, and we will never know the full extent of them. Especially the Norse Myths as the only written accounts came after the place was Christianized. It's hard to tell what is original and what is added by Christians. Their morals are much different to our own. Christians have a section of their book that says a father must sell his daughter to her rapist. Also a man can claim his wife unfaithful and the priest mixes up a poison to cause an abortion.
Myth is fucked up from our perspective and with our limited knowledge of it. You can deconstruct it with how it would be viewed today, but you'll never know what it was originally. Hell, a lot of what was in those Eddas could have been added to make the Gods look bad. You don't know.